1. Field of the Invention
This invention relates to optical display systems in which an image is reflected off a surface at a large bending angle, producing asymmetric aberrations in the reflected image, and more particularly to the use of such systems for helmet mounted visor displays that include optical systems to compensate the aberrations.
2. Description of the Related Art
Aircraft helmet visor display systems project images that provide necessary pilot information onto the helmet visor immediately in front of the pilot's eyes, allowing the pilot to see both the image reflected from the visor, and the outside scene, in a "heads up" mode. A typical helmet visor display system is described in U.S. Pat. No. 3,940,204, issued Feb. 24, 1976 to Withrington and assigned to Hughes Aircraft Company, the assignee of the present invention. The information image is provided from a cathode ray tube (CRT) mounted on the side of the helmet. The image is directed generally forward, and then deflected by a prism assembly onto the curved visor. To avoid obstructing the image with any part of the helmet or the pilot's head, the image is directed onto the visor at a substantial "bending angle" to a perpendicular from the center of the visor; the bending angle is typically about 55.degree.-60.degree.. This results in significant asymmetrical aberrations in the reflected image.
The aberrations produced with this asymmetrical system are quite complicated, and consist mainly of binodal astigmations and coma. In the past, a relay lens group has been introduced between the CRT and the visor, with the relay group tilted and decentered in an effort to produce equal and opposite aberrations and thus balance out (compensate) the aberrations that result from the asymmetries. The tilt angle of the CRT was also adjusted for this purpose. In addition, the inner surface of the visor (also referred to as a "combiner" since it combines both the projected information from the CRT and the outside scene) was implemented as a holographic surface upon which an asymmetric wavefront had been recorded to further reduce aberrations. The use of the relay group allowed the CRT to be kept small, while the combiner relayed the pupil of the wearer's eye to a location inside the relay group, thus allowing for a small size relay group. The efforts to reduce aberrations have not been wholly successful, however, and a significant residual aberration remained.
Astigmatism and coma are two significant aberrations which limit the image quality of many optical systems. For a symmetric optical system such as a camera lens, the astigmatism is quadradically proportional to the field of view, and coma is linearly proportional to the field of view. Therefore, at the center of the field, astigmatism and coma are zero. For an asymmetric optical system such as a high bending angle helmet visor display (HVD), the astigmatism and coma generated by the visor are much more complicated than that of a symmetric system. Astigmatism is generally characterized as having components that vary quadradically and linearly with the field of view, and also a constant component.
Therefore, there are generally two field points having zero astigmatism, which is different from a single field point of zero astigmatism for the symmetric optical systems. The term binodal astigmatism is used to characterize the two zero astigmatism field points. Coma for the asymmetric optical system includes a linear coma component that varies linearly with the field of view, and constant coma component that is constant over the field of view.
These various aberrations have either not been appreciated in the design of past compensation systems, or the systems were incapable of compensating all of them. In any event, past systems have attempted to eliminate only quadratic astigmatism, constant astigmatism and linear coma. This has resulted in an HVD with a small field of view, residual aberrations, small pupil sizes and limited image resolution.
Another approach has been to reduce the effective bending angle, and thereby eliminate the generation of asymmetric aberrations in the first place. This has been implemented by positioning a beam splitter directly in front of the wearer's eye to redirect the beam from the CRT, and thereby achieve a smaller bending angle and a wider field of view. A major problem with this approach is that it positions a distracting element very close to the wearer's eye, typically less than one-half inch away, and also obstructs the use of eyeglasses.
Another limitation of the prior approach is that holographic combiners have narrow bandwidths, typically restricting the system to a single color. This was not a particular problem with prior CRTs, which also had narrow bandwidths. However, more recent CRTs have a wider bandwidth, but the bandwidth of the overall system is limited to that of its narrowest bandwidth element, i.e., the holographic combiner. If the bandwidth limitation of the prior systems is removed by replacing the holographic combiner with a more conventional visor, however, both additional asymmetric aberrations and chromatic aberrations would result.